Galvanizing of reactive steels

ABSTRACT

An alloy for galvanizing steel comprises, by weight, aluminum in the amount of at least 0.001% to 0.007%, preferably 0.002 to 0.004%, tin in the amount of at least 0.5% to a maximum of 2%, preferably at least 0.8%, and one of an element selected from the group consisting of vanadium in the amount of at least 0.02%, preferably 0.05% to 0.12%, titanium in the amount of at least 0.03%, preferably 0.06% to 0.10%, and both vanadium and titanium together in the amount of at least 0.02% vanadium and at least 0.01% titanium for a total of at least 0.03%, preferably 0.05% to 0.15%, the balance zinc containing up to 1.3 wt. % lead. In another embodiment, the alloy comprises, by weight, aluminum in the amount of at least 0.001%, tin in the amount of 0.5% to 2%, and vanadium and nickel together in the amount of at least 0.02% vanadium and at least 0.02% nickel to a maximum of 0.15% vanadium and nickel collectively. Titanium may be added in an amount, by weight, of at least 0.01% titanium to a collective maximum of 0.2% vanadium, nickel and titanium. In a further embodiment, the alloy comprises, by weight, aluminum in the amount of at least 0.001%, tin in the amount of about 0.5% to about 2%, vanadium in the amount of 0.02 to 0.12%, and bismuth in the amount of 0.05% to 0.1%, the balance zinc containing up to 1.3 wt. % lead.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. application Ser. No.08/870,164, filed Jun. 6, 1997 now abandoned, for GALVANIZING OFREACTIVE STEELS, the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a galvanizing alloy and process and, moreparticularly, relates to a galvanizing alloy and an immersiongalvanization process adapted to control the undesirable effectsassociated with galvanizing reactive steels.

2. Description of the Related Art

The conventional process for hot dip galvanizing of low carbon steelscomprises pretreatment of said steels in a 20% to 30%, by weight,zinc-ammonium-chloride (ZnNH₄Cl) pre-flux, followed by immersion inmolten zinc or zinc alloy baths. The ‘normal’ or ‘N’ coating structureproduced on low reactivity steel by conventional hot dip galvanizingprocesses has well defined, compact alloy intermetallic layers. Thepredominant growth mode in this type of coating is by solid-statediffusion of iron and zinc, and thus well established intermetallicdelta and zeta layers control the rate of the galvanizing reaction. Thediffusion reaction rate decreases as the coating thickness increases,thus permitting predictable, consistent coverage. The normal coating hasa bright metallic luster.

Recent developments in the manufacture of low-alloy high-strength steelsinclude continuous casting. In the continuous casting process, it isnecessary to add elements that ‘kill’ or deoxidize the steel, i.e.,prevent gaseous products that produce porosity. Silicon is commonlyemployed for this purpose. The resulting steels generally containbetween 0.01% to 0.3%, by weight, silicon but may include up to or morethan about 0.5 wt % silicon and are known as ‘reactive steels’ orsilicon steels.

Phosphorus in the steel also affects reactivity, having an acceptedmeasure of reactivity that is approximately 2.5 times that of silicon.Thus, the silicon content plus 2.5 times the phosphorus content is knownas the effective silicon content of the steel.

Silicon steels that have high high reactivity pose problems to thegalvanizing process, producing thick, brittle and uneven coatings, pooradherence and/or a dull or marbled appearance. These coatings are knownas ‘reactive’ coatings. The high reactivity of the silicon steels alsocauses excessive zinc consumption and excessive dross formation.

Silicon released from the steel during galvanizing is insoluble in thezeta layer, which creates an instability in that layer and producesthick, porous intermetallic layers. The microstructure is characterizedby a very thin and uneven delta layer overlaid by a very thick andporous zeta layer that allows liquid bath metal to react near the steelinterface during the entire immersion period. The result is a lineargrowth mode with immersion time that allows the formation of excessivelythick coatings. These undesirably thick coatings are generally veryrough, brittle, and dull in appearance.

Steels with silicon levels between 0.05 to 0.15 (i.e. around the“Sandelin Peak” area). may also develop a ‘mixed’ reactivity or ‘M’coating, which is characterized by a combination of reactive andnon-reactive areas on the same steel that is believed to be the resultof differences in localized silicon levels on the surface of the steel.

It is known in the prior art to control reactivity by producing bathtemperature and immersion time at a rate inversely proportional to thesilicon content of the steel. Lower bath temperatures, on the order of430° C., and reduced immersion times tend to control the reactivity ofhigh silicon steels. However, using low bath temperatures and reducedtimes on low silicon steels produces unacceptably thin coatingthicknesses. Thus, the galvanizer must know the silicon content of thesteel beforehand and adjust the hot dip parameters accordingly. Thisapproach cannot be implemented if steel reactivity is not known or ifcomponents to be galvanized comprise parts of different reactivitieswelded together. With low-temperature galvanizing, productivity can bepoor because of the need to increase immersion times.

It is also known to control steel reactivity by adding alloy elements tothe zinc galvanizing bath. One such addition is nickel in a processknown as the Technigalva™ (or Nickel-Zinc) process A nickel content of0.05 to 0.10% by weight in the zinc bath effectively controls reactivesteels having up to about 0.2% by weight silicon content. For steelshaving silicon levels above approximately 0.2 wt. %, this nickel-zincprocess is not effective and thus it is only a partial solution to thereactive steel galvanizing problem. Normal steels of low reactivity,when galvanized by the nickel-zinc process, pose the same difficulty asseen in low temperature galvanizing in that coating thickness may beunacceptably thin. With this process, it is thus preferred that thegalvanizer know the reactivity of the steel beforehand and adjustgalvanizing conditions accordingly, both of which are difficult toaccomplish in practice. Under some conditions, this process alsoproduces dross that tends to float in the bath and be drawn out on theworkpiece, producing unacceptable coatings.

Another alloy used to control reactivity is that disclosed in FrenchPatent No. 2,366,376, granted Oct. 27, 1980, for galvanizing reactivesteels, known as the Polygalva™ process. The alloy comprises zinc ofcommercial purity containing, by weight, 0.1 to 1.5% lead, 0.01 to 0.05%aluminum, 0.03 to 2.0% tin, and 0.001 to 2.0% magnesium.

U.S. Pat. No. 4,439,397, granted Mar. 27, 1994, discusses theaccelerated rate at which the magnesium and aluminum are consumed orlost in this Polygalva™ process for galvanizing steel. Procedures arepresented to overcome the inherent difficulty in replenishing deficientaluminum or magnesium in the zinc alloy galvanizing bath. The processhas serious limitations in that the steel has to be meticulouslydegreased, pickled, pre-fluxed, and oven-dried to obtain good qualityproduct free of bare spots. Thus, in most cases, new high-qualityinstallations are usually required.

U.S. Pat. No. 4,168,972, issued Sep. 25, 1979, and U.S. Pat. No.4,238,532, issued Dec. 9, 1980, also disclose alloys for galvanizingreactive steels. The alloys presented include variations of thePolygalva™ alloy components of lead, aluminum, magnesium, and tin inzinc.

It is known in the prior art that aluminum included in the galvanizingbath reduces the reactivity of the high silicon steels. A process knownas the Supergalva™ process includes an alloy of zinc containing 5 wt. %aluminum and requires a special flux and double dipping not generallyaccepted by commercial galvanizers.

Co-pending U.S. patent application Ser. No. 08/667,830 filed Jun. 20,1996 now abandoned, the disclosure of which is incorporated herein byreference, describes a new alloy and process for controlling reactivityin steels with silicon content up to 1 wt. %. The alloy comprises zincof commercial purity containing, by weight, one or both of vanadium inthe amounts of at least 0.02% to 0.04% and titanium in the amounts of atleast 0.02% to 0.05%.

It is a principal object of the present invention to provide a processand alloy to effectively control reactivity on a full range of steels,including low and high silicon steels. The process should also producecoatings of acceptable and uniform thickness over the full range ofsteels.

Another object of the invention is to provide an alloy and process thatuses standard galvanizing equipment operated under normal conditions forgalvanizing steels of mixed reactivity without the need to adjust forvariations in steel chemistry.

SUMMARY OF THE INVENTION

The disadvantages of the prior art may be substantially overcome byproviding a new galvanizing process and alloy that can be readilyadapted to standard hot-dip galvanizing equipment.

The process of the present invention for galvanizing steel. includingreactive steels, comprises immersing the steel in a molten bath of azinc alloy comprising, by weight, aluminum in the amount of at least0.001% to 0.007%, preferably 0.002% to 0.004%, tin in the amount of atleast 0.5% to a maximum of 2%, preferably at least 0.8%, and one of anelement selected from the group consisting of vanadium in the amount ofat least 0.02%, preferably 0.05% to 0.12%, titanium in the amount of atleast 0.03%, preferably 0.06% to 0.10%, and both vanadium and titaniumtogether in the amount of at least 0.02% vanadium and at least 0.01%titanium for a total of at least 0.03%, preferably 0.05% to 0.15%, thebalance zinc containing up to 1.3 wt. % lead.

Also in accordance with the present invention is an alloy forgalvanizing steel that comprises, by weight, aluminum in the amount ofat least 0.001% to 0.007%, preferably 0.002 to 0.004%, tin in the amountof at least 0.5% to a maximum of 2%, preferably at least 0.8%, and oneof an element selected from the group consisting of vanadium in theamount of at least 0.02%, preferably 0.05% to 0.12%, titanium in theamount of at least 0.03%, preferably 0.06% to 0.10%, and both vanadiumand titanium together in the amount of at least 0.02% vanadium and atleast 0.01% titanium for a total of at least 0.03%, preferably 0.05% to0.15%, the balance zinc containing up to 1.3 wt. % lead.

In another embodiment of the invention, the alloy comprises, by weight,aluminum in the amount of at least 0.001%, tin in the amount of 0.5% to2%, and vanadium and nickel together in the amount of at least 0.02%vanadium and at least 0.02% nickel to a maximum of 0.15% vanadium andnickel collectively. Titanium may be added in an amount, by weight, ofat least 0.01% titanium to a collective maximum of 0.2% vanadium, nickeland titanium. In a further embodiment of the invention, the alloycomprises aluminum in the amount, by weight, of at least 0.001%, tin inthe amount of about 0.5% to about 2%, vanadium in the amount of 0.02 to0.12%, and bismuth in the amount of 0.05% to 0.1%, the balance zinccontaining up to 1.3 wt. % lead.

BRIEF DESCRIPTION OF THE DRAWINGS

The process of the invention and the alloy produced thereby will now bedescribed with reference to the following drawings:

FIGS. 1-3 are graphs illustrating galvanized coating thickness of avariety of galvanizing coatings on steel surfaces having a siliconcontent ranging from 0 to 1.0 wt. % under conditions of eight-minuteimmersion at 450° C., FIG. 1 being a graph showing average coatingthickness versus silicon content in a galvanizing bath of Prime Wegtern(PW) zinc with tin and vanadium, FIG. 2 being a graph showing averagecoating thickness versus silicon content in a galvanizing bath of PWzinc with tin and titanium, and FIG. 3 being a graph showing averagecoating thickness versus silicon content in a galvanizing bath of PWzinc with tin and both vanadium and titanium together.

FIG. 4 is a graph illustrating kettle material weight losses for avariety of galvanizing alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1-3, curve 10 typifies the variation ofthickness in microns on a steel surface of a coating of zinc ofcommercial purity, such as conventional Prime Western (PW), as afunction of the silicon content of the steel. The term “commercialpurity” used herein will be understood to include Prime Western, HighGrade, and Special High Grade zinc. Under these conditions of bathtemperature (450° C.) and immersion time (8 minutes), the thickness ofzinc coating peaks at a thickness of about 260 microns at a siliconcontent of about 0.15 wt. %, decreases to a thickness of about 175microns at a silicon content of about 0.2 wt. %, and then increases to amaximum thickness of about 375 microns at a silicon content of about 0.5wt. %, decreasing in thickness slightly to a silicon content of 1.0 wt.%. This curve 10 will be recognized as being very similar to thewell-known Sandelin Curve. The compositions of the steels used arelisted in TABLE I below.

TABLE I STEEL COMPOSITIONS: 1995 TRIALS Steel Si Alloy MTL ChemicalComposition (%) wt. Equiv- # Heat # Si P C S Mn Al alent* 1 95-18 0.021<0.006 0.11 .0071 0.59 0.019 0.021 95-20 0.019 0.11 .0051 0.76 0.0350.019 2 95-4a 0.15 <0.006 0.10 .0037 0.71 0.015 0.15 95-4b 0.15 0.10.0026 0.70 0.016 0.15 3 95-4c 0.21 <0.006 0.10 .0029 0.73 0.007 0.2195-4d 0.21 0.11 .0038 0.73 0.005 0.21 95-51 0.19 0.13 .0073 0.73 0.0460.19 4 95-21a 0.29 <0.006 0.10 .0030 0.70 0.035 0.29 95-21b 0.30 0.10.0028 0.71 0.046 0.30 5 95-28 0.32 <0.006 0.09 .0069 0.76 n.a. 0.3295-42 0.36 0.12 .0067 0.83 0.032 0.36 6 95-21c 0.46 <0.006 0.10 .00300.73 0.037 0.46 95-21d 0.46 0.10 .0029 0.73 0.036 0.46 7 95-22a 0.51<0.006 0.09 .0036 0.68 0.040 0.51 95-22b 0.51 0.10 .0032 0.68 0.042 0.518 95-22c 0.99 <0.006 0.09 .0031 0.71 0.022 0.99 95-22d 0.98 0.09 .00310.71 0.022 0.98 9 95-23a .019 0.02 0.09 n.a. 0.66 0.010 0.07 95-23b .0180.02 0.09 0.65 0.010 0.07 10  95-39 .031 0.050 0.10 .0071 0.80 0.0360.16 95-40 .023 0.055 0.09 .0072 0.71 0.047 0.16 Si equivalent = Si +2.5 P n.a. = not available

In accordance with ASTM Standards, e.g., the ASTM A-123 Standard (610g/m² or 86 microns for 3.2 to 6.4 mm thick steel plate), a uniformcoating thickness of about 100 microns is desired in order to meetminimum thickness requirements while avoiding the expense and waste ofthick coatings. Also, excessive thickness of zinc coatings on reactivesteels and steels of mixed reactivity due to high or variable siliconcontents usually produce rough, porous, brittle, and generally unsightlycoatings that can have poor adherence to the underlying steel surface.

It is generally accepted that the addition to the galvanizing bath ofstrong silicide formers may neutralize the influence of silicon inreactive steels. It has been found that vanadium alone is an effectivealloying element for reducing the reactivity of silicon steels with upto 0.25 wt. % Si. Vanadium in the bath is believed to combine with thesilicon to form vanadium suicides as inert particles that becomedispersed in the zeta layer. The silicon-free iron can then react withzinc to form a very compact and smooth layer that prevents liquid bathmetal from reaching the delta layer. In essence, the vanadiumeffectively suppresses reactivity by stabilizing the growth of the zetalayer in the coating, which controls the growth rate by a diffusionprocess.

It has been found that tin is also an effective element for reducing thereactivity of steels. Tests have shown that a galvanizing bathcontaining 2.5 wt. % to 5 wt. % tin can control reactivity in steelswith up to 1 wt. % silicon content. However, tests have also shown thattin in amounts greater than 2 wt. % react rapidly with the galvanizingkettle wall steel at galvanizing temperatures. When the tin level in thegalvanizing bath is below 2 wt. %, the reaction with the kettle steelproceeds at a slow rate, comparable to that of the commercial gradezinc. However, when the level of tin in a galvanizing bath is 2 wt. %,the presence of tin controls reactivity in steels with only up to 0.3wt. % silicon.

The presence of at least 0.02 wt. % vanadium, preferably 0.05 wt. % to0.12 wt. %, the solubility limit of vanadium, in combination with 0.5wt. % to 2 wt. % tin, controls reactivity in steels having up to 1 wt. %silicon. Tests have shown that, in galvanizing baths containing 1 to 1.2wt. % tin, 0.002 wt. % aluminum, and the balance zinc of commercialpurity containing 0.8 wt. % lead, the presence of 0.05 wt. % to 0.08 wt.% vanadium effectively controls reactivity to varying degrees in steelshaving silicon contents up to 1 wt. %, as shown by the Sn—V curves 11and 12 in FIG. 1.

Zinc of commercial purity, such as conventional Prime Western, containsup to 1.3 wt. % lead, typically about 0.8% lead. However, otheravailable grades of zinc such as High Grade and Special High Grade havelower lead contents. There is a growing tendency to reduce and eliminatethe presence of lead in galvanizing because of environmental, health andsafety concerns. It has been observed that bare spots in galvanizedcoatings can be produced from galvanizing baths without lead or withreduced lead contents at lower levels of tin, about 1 wt. % tin with0.05 wt. % vanadium and 0.002 wt. % aluminum, on steels having lowersilicon contents.

It has been found that the addition of 0.05 wt. % to 0.5 wt. %,preferably 0.05 wt. % to 0.1 wt % bismuth, to Zn—Sn—V alloys containing0.5 wt. % to 2 wt. % tin, 0.05 wt. % to 0.12 wt. % vanadium, 0.001 wt. %to 0.007 wt. % aluminum, the balance zinc, results in uniformly thickbright galvanized coatings having spangling and free of bare spots. Thepresence of bismuth is particularly beneficial for tin contents in therange of 1 wt. % to 1.5 wt. %.

In another embodiment of the process of the present invention, titaniumis used in place of vanadium. The presence of at least 0.03 wt. %titanium, preferably 0.06 wt. % to 0.1 wt. %, in combination with 0.5wt. % to 2.0 wt. % tin, controls reactivity in steels having up to about0.5 wt. % silicon. Iin a galvanizing bath containing 1.9 wt. % tin,0.002 wt. % aluminum, and the balance zinc of commercial purity, thepresence of 0.06 wt. % to 0.10 wt. % titanium effectively controlsreactivity to varying degrees in steels having silicon contents up toabout 0.5 wt. %, as shown by Sn—Ti curve 13 in FIG. 2. Increasing thetitanium content in the galvanizing bath to 0.1 wt. % does not increasethe maximum silicon level controlled, as seen by Sn—Ti curve 14 in FIG.2.

However, the addition of titanium to the bath forms a ternary Zn—Fe—Tiintermetallic that increases the amount of dross and ash duringgalvanizing and contributes to high rates of titanium consumption ordepletion in the bath. It also adversely affects the appearance of thegalvanized coating by eliminating the distinctive large spangle formedwith the tin-vanadium alloy that most galvanizing customers favor.

Small amounts of titanium added to the tin-vanadium alloy as asubstitute for a portion of the vanadium can be used to lower the levelof vanadium in the alloy, without the adverse effects of the hightitanium-tin alloy. The presence of at least 0.02 wt. % vanadium and atleast 0.01 wt. % titanium, preferably 0.05 wt. % to 0.1 wt. % vanadiumand titanium collectively, controls reactivity in steels having up to 1wt. % silicon. In a galvanizing bath containing 1 wt. % tin, 0.002 wt. %aluminum, and the balance zinc of commercial purity, the presence of0.06 wt. % vanadium and 0.02 wt. % titanium effectively controlsreactivity in steels having silicon contents up to 1 wt %, as shown bySn—V—Ti curve 16 in FIG. 3. Reducing the vanadium content in the alloymay be desirable in some cases to offset the high cost of vanadium ascompared to titanium.

Another embodiment of the alloy composition of the invention has utilityin zinc-nickel alloy baths comprises aluminum in the amount of at least0.001 wt. %, tin in the amount of about 0.5 wt. % to about 2 wt. %, andvanadium with nickel in the amount of at least 0.02 wt. % vanadium andat least 0.02 wt. %, preferably 0.05 wt. % to 0.1 wt. %, nickel, to amaximum of 0.15 wt. % vanadium and nickel collectively. The alloycompositions and the process of the invention will now be described withreference to the following illustrative examples.

EXAMPLE 1 Long Term Immersion Experiments of Kettle Steel in Zinc AlloyBaths to Determine Rate of Attack on the Steel and Maximum AllowableLimit for Tin in the Galvanizing Alloys

Four alloys were prepared, and samples from kettle steel were immersedin each alloy for a period of about 11 days at a temperature of 480° C.This immersion temperature was about 30° C. higher than the normalgalvanizing bath temperature to accelerate the reaction of the alloyswith the kettle steel samples. All the baths were saturated with iron atthe start of the experiments, and an addition of 0.004 wt. % aluminumwas made. The baths were analyzed during the 11-day trial period, andadditions were made as needed to maintain the nominal bath compositions.The four alloy compositions are listed in TABLE II below.

TABLE II Alloy Alloy Composition % wt No. Designation Sn V Ti Ni 1 PW —— — — 2 Sn—Ni 2.5 — — 0.05 3 V—Ti — 0.04 0.05 — 4 Sn—V 1.0 0.05 — —

The composition of alloy No. 2 (Sn—Ni) is a high tin alloy. Thecomposition of alloy No. 3 (V—Ti) is included in U.S. patent applicationSer. No. 08/667,830. The composition of alloy No. 4 (Sn—V) is anembodiment of the alloy of the present invention.

Fifty-kg melts were prepared in a SiC crucible that was heated in aradiant tube furnace. Four steel samples measuring 32×51×25 mm wereimmersed in each alloy bath. Analysis of the kettle steel showed itscomposition to contain, by weight, 0.09 wt. % carbon, 0.02 wt. %silicon, 0.006 wt. % phosphorus, and 0.27 wt. % manganese. The sampleswere machined to remove surface scale, degreased with acetone, pickledin hydrochloric acid, weighed, measured, and pre-fluxed in ZnNH₄Cl priorto immersion in the alloy baths.

The samples were removed after approximately 2, 4, 7 and 11 daysimmersion, and the coatings on the samples were removed by immersionfirst in hot sodium hydroxide solution and then in cold hydrochloricacid solution, and re-weighed. The differences in weight loss weredivided by the initial surface areas of the samples to determine weightloss in grams/mm² of unit area. The results are shown in the graph ofFIG. 4 as weight loss in g/mm² versus the immersion period in hours.

The curves in FIG. 4 show that the weight losses for alloy baths No. 3(V—Ti curve) and No. 4 (Sn—V curve) are comparable to that observed forbath No. 1 (PW curve). The weight loss from alloy bath No 2 (Sn—Nicurve) after 150 hours is about six times as great as the others (Nos.1, 3 and 4). More importantly, the slope of the No. 2 alloy curve isvery steep, indicating that the reaction with the steel follows a rapidlinear growth with immersion time that results in the formation ofexcessively thick coatings.

An additional PW melt was prepared and additions of tin were made at 0.2wt. % increments, from 0.5 wt. % to 2.5 wt. % tin. Kettle steel sampleswere immersed at 480° C. and inspected after 24 hours and 48 hours. Ifno evidence of excessive coating growth was observed after 48 hours, thetin content in the bath was increased by 0.2 wt. %. When evidence ofexcessive growth was first observed, the tin content in the bath wasreduced by 0.2 wt. %, and steel samples were immersed for a period ofabout two weeks to ensure that the coating growth rate was normal. Fromthese experiments, it was determined that when the tin content in thebath exceeded 2 wt. %, the abnormal or excessive growth rate began tooccur.

EXAMPLE 2 Galvanizing Trials

Ten alloys were prepared for laboratory-scale galvanizing trials. Thealloying additions were made to PW grade zinc. The typical compositionof PW zinc is shown in TABLE III below.

TABLE III COMPOSITIONS OF PW ZINC Element PW (%) Element PW (%) Pb 0.80 Cd 0.0019 Fe 0.009  Ca  0.00005 Al 0.004  Zr — Si 0.0004 Cu 0.0032 Mn0.007  Mg  0.00002 Ni 0.0005 As — Cr 0.001  B — Ti 0.0002 Ga  0.00005 V— Ge 0.0003 Sn 0.0001 In — Sb 0.0004 Ti 0.0002 Bi 0.002  Zn bal. Ag0.0004 — —

The experimental baths listed in TABLE IV below all were saturated withiron, and appropriate amounts of a 5 wt. % aluminum master alloy wereadded to maintain a 0.002 wt. % (brightener) aluminum level in the bath.The tin additions were made with high purity tin ingot, the vanadiumadditions with a Zn-2.3 wt. % V master alloy, and the titanium additionswith a Zn-4 wt. % Ti master alloy.

TABLE IV BATH ALLOY COMPOSITIONS Bath % Element Trial No. Designation SnV Ti 1 PW — — — 2 PW + Sn 1.8 — — 3 PW + Sn + V 1.8 0.04 — 4 PW + Sn + V0.4 0.12 — 5 PW + Sn + V 1.0 0.05 — 6 PW + Sn + V 1.2 0.08 — 7 PW + Sn +Ti 1.8 — 0.06 8 PW + Sn + Ti 1.8 — 0.10 9 PW + Sn + V + Ti 1.0 0.06 0.0210  PW + Sn + V + Ti 1.0 0.03 0.02 Note: All baths saturated in iron andcontain 0.002 wt % aluminum brightener.

A bench-scale line was set up to process the test samples consistently,using the following steps:

1. Degreasing: 0.25 g/cc NaOH solution at 70° C. with agitation for tenminutes

2. Rinse: tepid flowing water

3. Pickling: 15 wt. % HCl at room temperature, inhibited with Rodine™ 85(1:4000), for 20 minutes

4. Pre-flux: 20 wt. % Zaclon™ K (ZnNH₄Cl) at 60° C., for two minuteimmersion.

5. Drying: oven-drying for five minutes at 110° C.

Twenty-five kg melts were prepared in a SiC crucible that provided agalvanizing surface 150 mm in diameter. The crucible was heated in aradiant tube furnace to provide a galvanizing temperature of 450±2° C.The melt surface was skimmed prior to immersion and just before the testcoupons were withdrawn. The test coupons were dipped for eight-minuteimmersions at an immersion rate of 40 mm/sec and a withdrawal rate of 60mm/sec. The samples were air-cooled at room temperature, withoutquenching.

Hot-rolled low-carbon silicon-killed steel coupons, measuring 77 mm×39mm×3 mm, were used. The ten steel compositions, with silicon levelsranging from about 0.02 wt. % to 1 wt. %, are listed in TABLE 1, whichincludes the respective Si-equivalent or Si+2.5P level for the steelsthat takes into account the weighted effect of phosphorus as it relatesto the reactivity behavior of the steel.

The galvanized coatings produced in the experiments were evaluated bythe following methods:

Coating Appearance

The test coupons were photographed and classified under one of the threefollowing categories: Normal, Reactive or Mixed. A description for eachcategory of coating appearance is as follows:

Normal: The typical coating of a low-reactivity steel, usually brightand relatively smooth with visible spangle Mixed: The typical coating ofa reactive steel, usually matte-grey with no visible spangle Reactive:The typical coating of a steel that has both reactive and non-reactiveareas. The coating is usually very rough and varies from thin inlow-reactivity areas to thick in the reactive areas

Coating Thickness

Coating thickness measurements were made using an electromagneticthickness gauge.

The coating thickness results are presented in graph form in FIGS. 1-3and constitute the steel reactivity curves.

Metallography

Twenty-five-mm long pieces were cut from representative areas of thetest coupons and prepared by conventional metallographic techniques formicroscopic examination. All test samples were examined by opticalmicroscopy. Selected samples were examined with a scanning electronmicroscope (SEM), and energy dispersive x-ray micro-analysis (EDS) wasperformed on selected. samples as required. From these galvanizingtrials, the maximum effective steel silicon levels controlled by thevarious bath alloys were determined, the results being presented inTABLE V. As a reference. results of single element additions of tin,vanadium, titanium and nickel, obtained from past trials, are includedin TABLE V.

TABLE V MAXIMUM EFFECTIVE SILICON (ESi) LEVEL IN STEEL CONTROLLED BYALLOY ADDITION Bath Alloy Addition (%) Maximum PW Alloy Sn V Ti Ni ESi %Single 1.8* — — 0.09 0.20 Element — 0.12 — — 0.25 Addition — — 0.10 —0.30 Sn + Ti 1.8¹ 0.04 — — 0.50 Combination 0.4² 0.12 — — 0.50 1.0³ 0.05— — 0.50 1.2⁴ 0.08 — — 1.0 Sn + Ti 1.8 — 0.06 — 0.5 M 1.8 — 0.10 — 0.5 MSn + V + Ti 1.0 0.06 0.02 — 1.0 1.0 0.03 0.02 — 0.5 Notes: ¹High Sn -Low V ²Low Sn - High V ³Preferred composition for 0.5% ESi ⁴Preferredcomposition for 1.0% ESi M Marginal control with various amounts ofmixed reactivity and heavier coatings than when fully controlled.

The results show that, as a single element addition, the maximumeffective silicon level controlled is about 0.3 wt. %. When tin andvanadium are combined, 0.5 wt. % effective silicon can be controlledwith a minimum level of 0.04 wt. % vanadium and a tin level of 1.8 wt. %(which is near the maximum allowable level), and with a minimum level of0.4 wt. % tin and a 25 0.12 wt. % vanadium level. A preferredcomposition for controlling the 0.5 wt. % Si level is 1.0 wt. % tin with0.05 wt. % vanadium. The 1.0 wt. % effective silicon can be controlledwith a preferred composition of 1.2 wt. % tin and 0.08 wt. % vanadium.

When tin is combined with titanium, the maximum effective silicon levelthat was controlled was 0.5 wt. %. even when the maximum allowableamount of 1.8 wt. % tin and an amount of 0.1 wt. % titanium were addedto the galvanizing bath.

When vanadium and titanium are added together, it is possible to controlthe 0.5 wt. % effective silicon with additions of 1.0 wt. % tin, 0.03wt. % vanadium, and 0.02 wt. % titanium, and the 1 wt. % effectivesilicon level with additions of 1.0 wt. % tin, 0.06 wt. % vanadium, and0.02 wt. % titanium. The addition of titanium to the tin and vanadiumalloy allows for a reduction in the amount of vanadium needed to controlat both the 0.5 wt. % and 1.0 wt. % effective silicon levels.

EXAMPLE 3 Addition of Bismuth

Trials were conducted on 77 mm×39 mm×3 mm low silicon steel coupons thatwere pretreated by an acetone rinse and scrubbing, pickling in 15% HClsolution for 10-15 minutes, preflux of ZACLON K™ (20° Be) for 2 minutesat 70° C., and oven-dried at 100° C. for 5 minutes.

The coupons were galvanized by immersion for 4 minutes in zinc alloybaths of Special High Grade 25-kg melt saturated with iron andcontaining 0.004 wt. % aluminum, 1 wt. % tin, 0.05 wt. % vanadium, andvarying amounts of bismuth at a temperature of 450° C.

The test results are shown in TABLE VI.

TABLE VI BATH ALLOY COMPOSITIONS-SHG + Sn + V + Bi % Element Trial No.Sn V Bi Observations 1 1.0 0.05 Severe bare spots and small spangling 21.0 0.05  0.05 Substantially complete elimination of bare spots, andlarger spangling 3 1.0 0.05 0.1 Free of bare spots and larger spangles 41.0 0.05 0.2 Free of bare spots and very larger spangles 5 1.0 0.05 0.5Free of bare spots and very larger spangles Note: all baths contain0.004 wt. % aluminum brightener

Note: all baths contain 0.004 wt. % aluminum brightener

The presence at least 0.05 wt. % bismuth was found to be effective inobviating bare spots and in enhancing spangling of the galvanizedcoating. An upper limit of bismuth of 0.1 wt. % bismuth was foundeconomically viable; amounts in excess of 0.1% up to 0.5% did notimprove the quality of coating.

The invention provides several important advantages. Galvanized coatingsproduced in accordance with the invention are complete and uniform andof desired thickness on low and high silicon steels, including steelhaving silicon contents from 0.01 wt. % to at least 0.5 wt. %. Thecoatings produced also have a bright metallic luster. The process can beeasily adapted to conventional galvanizing production equipment usingnormal galvanizing temperatures and immersion times.

It will be understood, of course, that modifications can be made in theembodiment of the invention illustrated and described herein withoutdeparting from the scope and purview of the invention, as defined by thefollowing claims.

What is claimed is:
 1. An alloy for galvanizing steel consistingessentially of, by weight, aluminum in the amount of 0.001% to 0.007%,tin in the amount of 0.5% to 2%, and one of an element selected from thegroup consisting of vanadium in the amount of 0.02% to 0.12%, titaniumin the amount of 0.03% to 0.10%, and both vanadium and titanium togetherin the amount of at least 0.02% vanadium and at least 0.01% titanium fora total of 0.03% to 0.15% vanadium and titanium collectively, andoptionally bismuth in the amount of 0.05% to 0.5%, the balance zinc ofcommercial purity containing up to 1.3 wt. % lead.
 2. An alloy asclaimed in claim 1 further containing, by weight, vanadium in the amountof 0.05% to 0.12%.
 3. An alloy as claimed in claim 1 further containing,by weight, titanium in the amount of 0.06% to 0.10%.
 4. An alloy asclaimed in claim 1, in which the zinc alloy contains, by weight, atleast 0.03% vanadium and titanium when vanadium and titanium are presenttogether, said vanadium being present in the amount of at least 0.02%and said titanium being present in the amount of at least 0.01%, to amaximum of 0.15% vanadium and titanium collectively.
 5. An alloy asclaimed in claim 4 in which the vanadium and titanium are presenttogether, by weight, in the amount of at least 0.05%.
 6. An alloy forgalvanizing steel consisting essentially of, by weight, aluminum in theamount of 0.001% to 0.007%, tin in the amount of
 0. 5% to 2%, vanadiumin the amount of 0.02% to 0.12%, and optionally bismuth in the amount of0.05% to 0.5%, the balance zinc of commercial purity containing up to1.3 wt. % lead.
 7. An alloy as claimed in claim 6 further containing, byweight, bismuth in the amount of 0.05% to 0.1%.
 8. An alloy forgalvanizing steel consisting essentially of, by weight, aluminum in theamount of 0.001% to 0.007%, tin in the amount of 0.5% to 2.0%, andvanadium and nickel in the amount of at least 0.02% vanadium and atleast 0.02% nickel to a maximum of 0.15% vanadium and nickelcollectively, the balance zinc of commercial purity containing up to 1.3wt. % lead.
 9. An alloy for galvanizing steel consisting essentially of,by weight, aluminum in the amount of 0.001% to 0.007%, tin in the amountof 0.5% to 2.0%, vanadium in the amount of 0.02% to 0.12%, and bismuthin the amount of 0.05% to 0.5%, the balance zinc of commercial puritycontaining up to 1.3 wt. % lead.
 10. A process for galvanizing steel byimmersion in a zinc alloy galvanizing bath comprising: immersing thesteel in a molten bath of a zinc alloy consisting essentially of, byweight, 0.001% to 0.007% aluminum, 0.5% to 2% tin, and an amounteffective for reducing reactivity of the steel of at least one elementselected from the group consisting of 0.02% to 0.12% vanadium, 0.03% to0.10% titanium, and at least 0.02% vanadium and at least 0.01% titaniumfor a total of 0.03% to 0.15% vanadium and titanium collectively, andoptionally 0.05% to 0.5% bismuth, the balance zinc of commercial puritycontaining up to 1.3 wt. % lead.
 11. A process as claimed in claim 10,in which the zinc alloy contains, by weight, at least 0.05% vanadium.12. A process as claimed in claim 11, in which the zinc alloy contains,by weight, 0.05% to 0.12% vanadium.
 13. A process as claimed in claim10, in which the zinc alloy contains, by weight, at least 0.06%titanium.
 14. A process as claimed in claim 13, in which the zinc alloycontains, by weight, 0.06% to 0.10% titanium.
 15. A process forgalvanizing steel by immersion in a zinc alloy galvanizing bathcomprising: immersing the steel in a molten bath of a zinc alloyconsisting essentially of, by weight, 0.001% to 0.007% aluminum, 0.5% to2% tin, and vanadium and nickel in the amount of at least 0.02% vanadiumand at least 0.02% nickel to a maximum of 0.15% vanadium and nickelcollectively, and optionally 0.05% to 0.5% bismuth, the balance zinc ofcommercial purity containing up to 1.3 wt. % lead.
 16. A process forgalvanizing steel by immersion in a zinc alloy galvanizing bathcomprising: immersing the steel in a molten bath of a zinc alloyconsisting essentially of, by weight, 0.001% to 0.007% aluminum, 0.5% to2% tin; and vanadium, nickel, and titanium in the amount of at least0.02% vanadium, at least 0.02% nickel, and at least 0.01% titanium to amaximum of 0.2% vanadium, nickel, and titanium collectively, andoptionally 0.05% to 0.5% bismuth, the balance zinc of commercial puritycontaining up to 1.3 wt. % lead.
 17. A process for galvanizing steel byimmersion in a zinc alloy galvanizing bath comprising: immersing thesteel in a molten bath of zinc alloy consisting essentially of, byweight, 0.001% to 0.007% aluminum, 0.5% to 2.0% tin, 0.02% to 0.12%vanadium, and 0.05% to 0.5% bismuth, the balance zinc of commercialpurity containing up to 1.3 wt. % lead.
 18. A process as claimed inclaim 17, in which the molten zinc bath further contains, by weight,0.05% to 0.1% bismuth.